There's no need to return your iPhone 4 to them, no forms to fill out, Samsung just reached out to a few folks over twitter and asked for their contact details and a free Galaxy S was shipped to them.

The official word from Samsung UK:

"Recently there has been a real increase in online activity from consumers dissatisfied with some of our competitors' products. We decided to contact a cross section of individuals to offer them a free Samsung Galaxy S as a replacement, as we're confident that once people have the phone in their hands, they'll see how impressive it is for themselves."